1. Field of the Invention
The present invention relates to a signal transmission apparatus and a signal transmission method both for transmitting encoded audio information. More specifically, the present invention relates to a signal transmission apparatus and a signal transmission method both for transmitting a plurality of pieces of encoded audio information having a plurality of sampling frequencies of F or 1/N×F, which are encoded by the same encoding method, via a digital interface to a signal reception apparatus.
2. Description of the Related Art
Encoded audio information is transmitted from a signal transmission apparatus via a digital interface to a signal reception apparatus.
FIG. 7 is a diagram for explaining a conventional audio data processing apparatus 700, The audio data processing apparatus 700 comprises a signal transmission apparatus 701 for transmitting a transmission signal 704 including encoded audio information, a signal reception apparatus 703 for receiving the transmission signal 704, and a digital interface 702 for mediating the transmission signal 704 between the signal transmission apparatus 701 and the signal reception apparatus 703. The signal reception apparatus 703 may, for example, be a device for reproducing the transmission signal 704.
ISO/IEC61937 is a known standard for transmission and reception of encoded audio information conducted using the digital interface 702. In this standard, the signal transmission apparatus 701 transmits the transmission signal 704 including encoded audio information obtained by encoding a PCM (Pulse Code Modulation) signal having M samples per frame, in the form of a 16-bit-and-M-sample stereo signal. In this case, when the size of burst information including management information and encoded audio information is less than a block size of 16×M×2 bits, the signal transmission apparatus 701 writes stuffing information in the unused portion of the block (stuffing process), and specifically sets the entire unused portion to zero, thereby generating the complete block.
Typically, in the transmission signal 704 transmitted from signal transmission apparatus 701, synchronization word information appears in each block. The time between the start of a block indicated by certain synchronization word information and the start of another block indicated by the immediately following synchronization word information is herein referred to as a “repetition time”.
In the ISO/IEC61397 standard, it is assumed that a stream of MPEG2 audio having a low sampling frequency (LSF) [24 kHz, 22.05 kHz, or 16 kHz] is transmitted. In this case, the repetition time is twice as long as a repetition time when a stream of MPEG1 audio having a sampling frequency of 48 kHz, 44.1 kHz, or 32 kHz is transmitted.
A transmission clock of the digital interface 702 which is of a typical type as described in the ISO/IEC61397 standard is set, assuming that the sampling frequency of encoded audio information to be transmitted is 48 kHz, 44.1 kHz, or 32 kHz.
When different encoding methods are used to encode audio information as described above, the signal reception apparatus 703 needs to receive information about an encoding method in order for the signal reception apparatus 703 to correctly receive the transmission signal 704. In this standard, a value indicating that encoded audio information has been encoded by a different encoding method it defined in a bit field for data type information of the transmission signal 704 so as to notify the signal reception apparatus 703 that a different encoding method has been used.
In this case, as encoding methods for encoding audio information are changed, the repetition times of the transmission signal 704 are also changed. Therefore, if an encoding method which has been used to obtain encoded audio information is defined in data type information, the signal reception apparatus 703 can synchronize the transmission signal 704. Therefore, the signal reception apparatus 703 can appropriately process the transmission signal 704.
Now, it is assumed that encoded audio information, which has been obtained by the same encoding method but using a different sampling frequency, is transmitted via the digital interface 702 to the signal reception apparatus 703. The transmission clock of the digital interface 702 and the reception clock of the signal reception apparatus 703 are both constant. The digital interface 702 and the signal reception apparatus 703 are presumed to receive encoded audio information having a sampling frequency of 48 kHz.
Typically, the transmission clock of the digital interface 702 is equal to the reception clock of the signal reception apparatus 703. Therefore, in the following description, it is assumed that the transmission clock of the digital interface 702 is equal to the reception clock of the signal reception apparatus 703.
In the conventional art, the signal reception apparatus 703 cannot correctly receive encoded audio information having a sampling frequency other than an presumed sampling frequency unless the signal reception apparatus 703 is notified of a change in the repetition time of a transmission signal.
For example, when encoded audio information having a certain sampling frequency is transmitted and thereafter encoded audio information having a different sampling frequency is transmitted, the signal reception apparatus 703 cannot correctly receive the transmission signal 704 unless the signal reception apparatus 703 is notified of the change in the repetition time of the transmission signal 704.
Hereinafter, the conventional example will be described in more detail.
FIG. 8 is a diagram showing conventional structures of transmission frames including encoded audio information. A transmission frame is a part of the transmission signal 704. A series of transmission frames are output as the transmission signal 704 by the signal transmission apparatus 701.
FIG. 8(a) is a diagram showing a structure of a transmission frame 800 including encoded audio information 803. The encoded audio information 803 is one-frame data obtained by dividing encoded audio information having a sampling frequency of 48 kHz into frames. FIG. 8(b) is a diagram showing a structure of a transmission frame 850 including encoded audio information 853. The encoded audio information 853 is one-frame data obtained by dividing encoded audio information having a sampling frequency of 24 kHz into frames. In this case, an encoding method for encoded audio information having a sampling frequency of 48 kHz is the same as an encoding method for encoded audio information having a sampling frequency of 24 kHz, that is, an encoding method for the encoded audio information 803 is the same as an encoding method for the encoded audio information 853.
As shown in FIG. 8(a), the transmission frame 800 consists of one block. The transmission frame 800 includes a header portion 801 and a body portion 802. The header portion 801 stores management information 811. The body portion 802 stores data information 812. The management information 811 includes synchronization word information 821 and side information 822. The data information 812 includes encoded audio information 803. The data information 812 may include non “encoded audio information” 823.
The signal transmission apparatus 701 of FIG. 7 generates the transmission frame 800 from a signal input to the signal transmission apparatus 701, and outputs the transmission signal 704 including a series of transmission frames 800 to the digital interface 702.
The management information 811 is information for managing data stored in the body portion 802. As the data, the encoded audio information 803 and the non “encoded audio information” 823 are stored in the body portion 802. The management information 811 includes information indicating whether the data stored in the body portion 602 is valid. The synchronization word information 821 indicates the start of the block of the transmission frame 800. The synchronization word information 821 is used to obtain the repetition time between each transmission frame 800 sequentially transmitted. The side information 822 indicates whether the encoded audio information 803 is stored in the body portion 802. The non “encoded audio information” 823 indicates the absence of encoded audio information. The non “encoded audio information” 823 is used to make the size of the data information 812 constant.
As shown in FIG. 8(b), the transmission frame 850 consists of one block. The transmission frame 850 includes a header portion 851 and a body portion 852. The header portion 851 stores management information 861. The body portion 852 stores the data information 862. The management information 861 includes synchronization word information 871 and aide information 872. The data information 862 includes encoded audio information 853. The data information 862 may include non “encoded audio information” 873.
The signal transmission apparatus 701 of FIG. 7 generates the transmission frame 850 from a signal input to the transmission apparatus 701, and outputs the transmission signal 704 including a series of transmission frames 850 to the digital interface 702.
The management information 861 is information for managing data stored in the body portion 852. As the date, the encoded audio information 853 and the non “encoded audio information” 873 are stored in the body portion 852. The management information 861 includes information indicating whether the data stored in the body portion 850 is valid. The synchronization word information 871 indicates the start of the block of the transmission frame 850. The synchronization word information 871 is used to obtain a repetition time between each transmission frame 850 sequentially transmitted. The side information 872 indicates whether the encoded audio information 853 is stored in the body portion 852. The non “encoded audio information” 873 indicates the absence of encoded audio information. The non “encoded audio information” 873 is used to make the size of the data information 862 constant. The size of the body portion 852 is twice as large as the size of the body portion 802 of the transmission frame 800.
As described above, the encoded audio information 803 and the encoded audio information 853 are both one-frame data (i.e., data having the same number of samples) which are encoded by the same encoding method. However, since the sampling frequency of the encoded audio information 803 is different from the sampling frequency of the encoded audio information 853, a reproduction time of the encoded audio information 803 is different from that of the encoded audio information 853. Further, a time required for the transmission frame 800 including the encoded audio information 803 to be received by the signal reception apparatus 703 is different from a time required for the transmission frame 850 including the encoded audio information 853 to be received by the signal reception apparatus 703.
In the above-described case, the time required for the reception of the transmission frame 850 is twice as long as the time required for the reception of the transmission frame 800. The time required for the reproduction of the encoded audio information 853 is twice as long as the time required for the reproduction of the encoded audio information 803. Further, the repetition time of a transmission signal including the transmission frame 850 is twice as long as the repetition time of a transmission signal including the transmission frame 800.
As described above with reference to FIG. 8, when the sampling frequency of one type of encoded audio information is ½ of the sampling frequency of the other type of encoded audio information, the repetition time of a transmission signal carrying the one type of encoded audio information is twice as long as the repetition time of the other type of encoded audio information. Similarly, when the sampling frequency of one type of encoded audio information is 1/N of the sampling frequency of the other type of encoded audio information, the repetition time of a transmission signal carrying the one type of encoded audio information is N times as long as the repetition time of the other type of encoded audio information, where N is a natural number more than or equal to 2. When the sampling frequency of encoded audio information is decreased by a factor of 1/N, the size of a body portion received by the signal reception apparatus 703 is increased by a factor of N. As a result, the repetition time is also increased by a factor of N.
As described above, although the same encoding method is used, a repetition time varies depending on the sampling frequency of encoded audio information. If the transmission signal 704 is transmitted without notifying the signal reception apparatus 703 that a different repetition time is used, the signal reception apparatus 703 cannot be correctly synchronized with the transmission signal 704. Therefore, the signal reception apparatus 703 needs to be at least notified that a repetition time has been changed in order to correctly receive the transmission signal 704.
Alternatively, certain information relating to the sampling frequency of encoded audio information is defined in the management information in order to intermittently notify the signal reception apparatus 703 of the repetition time of the transmission signal 704. Conventionally, in the ISO/IEC61397 standard, when the same encoding method is used but a different repetition time is used, a different value is defined in data type information. The data type information relating to data information is included in the management information. However, when different data type information is defined for each sampling frequency, the number of data types to be written in data type information is increased. As a result, the bit field of the data type information cannot be effectively used, so that the bit field of the data type information becomes insufficient.